Automatic volume control



y 1935- 7.1. R. NELSON AUTOMATIC VOLUME CONTROL Filed Ma 27, 1951 Fig. 2

v smaq E Fig. 1

INVENTOR J4me A/asom BY W ATTORNIQEY Patented May 21, 1935 UNITED STATES PATENT OFFICE 7 2,001,825 AUTOMATIC VOLUME CONTROL James R. Nelson, Cambridge, Mass., assignor to Raytheon Production Corporation,

Newton,

Mass., a corporation of Delaware Application May 2'7, 1931, Serial No. 540,276 6 Claims. (01. 250-20) signal of low value is received, but only comes embodiment of my invention.

into operation when the signal exceeds a certain predetermined value.

The foregoing and other objects of my invention will be best understood from the following description of exemplifications thereof, reference being hadto the accompanying drawing, wherein Fig. 1 is a diagram of the circuits involved in a receiving set incorporating my present invention; and I i Fig. 2 is a diagram-of the circuits involved in a similar set, showing a slightly different modified One of the difiiculties met with in modern radio broadcast receiving is that the signal strength of a station as received by a broadcast receiving set often fluctuates quite widely. With the ordinary broadcast receiver, the set is adjusted so that at a certain value of signal strength, the output volume is of the desired value. .I-Iowever, when the magnitude of the received signal changes, the output volumealso changes, andis either too loud or too soft.' In accordance with my invention I eliminate this difiiculty by providing an amplifier with means whereby above a certain value of signal strength, the amplifying power thereof is varied inversely with the signal strength, and thus the. output level is maintained at a constant value. V

In order to understand the operation of my arrangement, it is necessary to consider certain theoretical aspects of the operation of aspace discharge device. In a space discharge device or tube, including a cathode, anode and control electrode, the theoretical voltage amplifying power of the tube, which is ordinarily represented as ,u is usually determined by the geometry of the tube. With the ordinary three-element tube a is practically constant. With other types of tubes, such as, for example, the screen-grid type of tube, a varies somewhatwith different voltages applied to the control grid and plate. The mutual conductance of a tube,'which is represented by gel, represents the real function ofthe device when used as an amplifier. The value of gmis obtained from the following equation:

g,,,= (Equation 1) v 1",,

where 1",) is the anode or plate circuit resistance. By multiplying. the signal voltage a; as applied to the grid bythe mutual conductance gm, we find the amount of alternating cur- .rent 2' set up in the plate circuit by the signal.

0 m; p (Equation 2) This is, based on the assumption that there is no external impedance in the plate circuit. Any such impedance will decrease the theoretical value of i obtained as above.

In accordance with my invention, I obtain a constant signal output. from an amplifier by causing the plate circuit resistance to vary in a predetermined manner in accordance with the variation in signal strength impressed on said amplifier. Intubes in which a also varies, I cause the variation of Tp to be substantially in excess of any corresponding variation in ,u. This variation in plate circuit resistance causes a corresponding variation in gm of the tube. I arrange my device so that gm, the mutual conductance of the tube, varies above a certain signal strength substantially inversely as the signal strength impressed on said tube.

When a negative bias is impressed upon the grid of a tube, there is introduced into the tube -a resistance to the electron fiow between the cathode and the plates. This manifests itself in an increase in the plate circuit resistance. As the negative bias on the gridincreases in value, the resistance of the plate-circuit likewise increases. I obtain my variation plate circuit resistance or gm, as called for above, by impressing a negative bias on the grid of an amplifying tube, and varying said negative bias in accordance with the signal strength received by my'device.

As stated above, the value of a in three-element tubes is not substantially affected by changes in grid bias while in tubes of the screen-grid type, a increases with increases in negative bias on the control grid. However, Tp of such screen grid tubes increases much' faster than does t with increases of control grid negative bias, Thus in each case the desired variation in gm is obtained with variations in said bias.

In Fig. 1 I have illustrated a radio receiving set comprising a radio frequency amplification stage I a detector stage 2, and an audiofrequency amplification stage 3. It will be understood that any variation in the number of radio and audio frequency amplification stages may ,be had;

Stage I comprises a. circuit consistingof 'a coil 5,

a variable condenser 6, and a tube 1. Oneside of the condenser E is connected to one end of the coil 5. The other side of the condenser is grounded on the conductor I 9. The other end ,of the coil 5 is also grounded by being connected through theradio frequency bypass condenser 4| to the conductor l9. An antenna circuit 4 is inductively coupled with the coil 5. The tube 1 includes a cathode 8, an anode or plate 9,

through the grid condenser i 8.

plate 15 in the output circuit of said plate.

voltage on the plate.

and a control grid H1. The control grid is connected to the non-grounded side of the coil 5 and condenser 6. A coil II is included in the output circuit of the plate 9, whereby the output of stage i is inductively coupled to the detector stage 2 by means of a coil l2. A radio frequency bypass condenser 54 may be connected between the cathode 3 and one end of the coil H, as

between the cathode and end of the resistance 2i opposite the plate I 5 in order to provide a return path for the audio frequency currents. A radio frequency bypass condenser may be provided to provide a bypass for any radio frequency currents which tend to pass through the output circuit of the plate 15. In the cathode return connection of the plate [5 is placed a resistance 22 which is shunted by a condenser 23 and a high resistance 23 in series. The end of coil 5 connected to the condenser M is also connected througha resistance 49 to the point between the condenser'23 and the resistance 23'.

' The cathode l4 and the sides of coil I2 and condenser l'l opposite the grid 15 are also connected to the conductor l9 which is grounded at 29. This conductor may be, for example, the grounded frame of a radio set.

' Audio stage 3 includes a tube 24, comprising 7 a cathode 25, a plate 26, and a control grid 21. The variations in potential on the plate i5 of the detector stage 2 are impressed on the grid 27 of the audio stage. 3 by means of a condenser 28. An output device'29, which may be, for example, aloud speaker, isconnected in the output circuitof the plate 261 In the cathode return connection of the plate .26 is placed resistance 30 bridged by a condenser 3!. The condenser Si is large enough so as not to offer any considerable impedance to audio frequency currents. The

power which can be dissipated in the plate 25 is a limited value, and the power so dissipated is a maximum under no signal conditions. By inserting the resistance 30, thecurrent through the plate 26 is kept at a safevalue under no signal conditions, even with the use of a fairly high This arrangement permits the use of a higher voltage on the plate 25 than ,could otherwise be used. and results in an increase in the power obtainable from the tube 24. The

condenser 28 is connected through a resistance 42 to theend of resistance 3!! opposite the cathode 25. The grid is connected by a variable tap as to any desiredpoint on resistance 42.

The circuits are supplied with the proper direct current potentials from a voltage divider, which consists of a resistance 32 connected across the source of direct current, which may be, for example, a battery 33.

,lI'he plate 26 is connected through the output device 29 to one terminal 35. of the resistance 32. The cathode 25 is connected through theresistamine 30 to the other terminal 35 of said resistance a 224, type.

The plate I5 is provided with the proper potential from a tap 3'6 on the resistance 32. This potential is preferably somewhat lower than that applied to the plate 36. An audio frequency choke 4? may be placed in the connection to plate l5, as shown, whereby the audio frequency currents are prevented from flowing through the resistance 32. Instead they return to the cathode through the condenser 45. The cathode I l of the tube It is connected as is the cathode 25 to the terminal 35.

The plate 9 of the tube I is also provided with a suitable potential from a tap 3? on the resistance 32. This potential is preferably lower than that applied to either plates E5 or 26. The cathode 5, however, instead of being connected directly to the terminal 35 is connected to a tap 38 spaced from said terminal 35 on the resistance 32.

Although I have described above and have illustrated diagrammatically in the drawing threeelement tubes in each of the stages, I wish it to be understood that various types of tubes may be used in these stages. In stage i, for example,

tube I may consist of a triode of the 227 type, or

it may consist of a screen-grid tetrode of the 224 type. If a screen-grid tube is used, the screen grid itself is maintained at a definite potential with respect to the cathode in accordance with the usual practice. tube of the 227 type could be used, although I prefer to use a screen-grid detector tube of the The connections of the screen, grid in this type would likewise be made in the conventional manner. In stage .i a power amplifier tube of the 245 type could be employed. Howevena pentode tube of the 24'? type could be utilized in this stage. Although I have men tioned specific tubes above, I wish it to be understood that any type of tube which has the necessary characteristics could be employed in any of the stages.

1 The operation of the device, as described above, can 'be best understood from a consideration of the theory of operation of a tube connected as is the device it in Fig. 1. In such a tube the condenser E8 and resistance It serve to impress a negative potential upon the grid US, which potential varies in accordance with the average value of themagnitude of the radio frequency signal impressed on said grid. Thus, as the magnitude of the signal increases, the negative bias on the grid IE will increase. Asv is well known, an increase in negative bias on the grid will cause a decrease in the current flowing to the plates. The constants of the condenser i8 and resistance 63 are so chosen that when a modulated signal is received, the variations in voltage on the grid l6 closely follow the modulation frequency carried by the radio frequency signal wave.

In addition to the audio frequency variation, the signal is sometimes subject to a slower variation in signal strength. If we compare an unmodulated radio frequency wave and a wave of the same intensity carrying an audio frequency modulation, we find that the average magnitude of each of said waves over a period of time which is considerably greater than the period of the audio frequency, do not differ from each other to any considerable degree. Changes in this average value occur only with changes in signal strength. The changes in signal strength with which we are concerned usually OCCLll at a rate which is considerably slower than that of any In stage 2, a three-element audio frequency which may be received. Therefore we see that the negative bias on the grid 6 will not only follow the audio frequency modulation, but will also vary in magnitude in accordance with the slower variations-in signal strength. The device, as shown in Fig. 1, utilizes this slower variation in grid bias to compensate for the variation in the strength of the signal received in the set. This occurs in the following manner.

Let the values of certain currents and -resistances in Fig. 1 be indicated by the following symbols: I r Y V1o=grid bias in grid id R1=resistance between and 38 Rz resistance of 22 i9=currentin circuit of plate 9 i15=current in circuit of plate l5 i1e=current through R2 im current through R1 i1i=current through 32 between points 31 and'38 In each case i is the average of the current flowing through its respective circuit, said average being taken over a period of time considerably in excess of the audio frequency period, Such an average current can be considered as the direct current component of the current flowing in its respective circuit. With no signal received by the set, the voltage between the grid l0 and the cathode 8 of the space discharge device I is The values of the resistance R1 and R2 and the ther constants of the circuits are so chosen that l with no signal received by the set, imRi is greater an absolute value than iisRz to such an extentare adjusted so that their respective circuits are tuned-to the desired signal frequency being received in the antenna circuit 4. This signal is amplified in-stage l and the voltage variations produced thereby are impressed on the grid l6 of the space discharge device I3 by means of the coil l2. Due to the action of the grid condenser and the resistance [8 connected across it, the grid bias 'on the grid IE will vary in accordance with the audio frequency modulation of the signal and also, in accordance with the slower changes in signal strength as described above. The'tube acts so as to produce a. corresponding variation in current in the plate IS in an amplified degree. As has been indicated above, the variations in current in the plate circuit take place in such a manner that the current decreases upon an increased negative bias on the grid. Thus an increase in signal strength will result in a decrease in the direct current component 1'15 of the plate current. Since the value of current flowing through R2, due to the grid his negligible with respect to that, due to 1'15, for all practical purposes it can'be said that 15 is equal to 1 1 6. By referring to Equation 3, it will be seen that a decrease in the. value of in; will result in an increase of the negative bias V10 on the grid In. This increase in negative bias produces an in crease in plate resistance r and a corresponding decrease in on of the tube I, in accordance with the explanation above.

Thus it will be seen that when the signal voltage Cg applied to the tube 1 increases, the value of gm of that device decreases. Since from Equation 2, i is the product of gm and eg, the fact thatthey vary in opposite directions produces a tendency for the amount of alternating current ip set up in the circuit of plate 9 to remain constant. Therefore, the strength of the signal output in the amplifier stage I as impressed on the detector 2 tends to remain practically constant. Accordingly the output of detector 2 and consequently of-the set likewise'tends to remain at a constant level.

Since the audio frequency is subjected to considerable variations in amplitude and since it is desired to reproduce these variations, it is desirable to prevent the audio frequency current from passing through the resistanceRz and varying the bias on the grid 10. In Fig. 1 I accomplish this result first by providing a condenser which allows the audio frequency to return from'the plate to the cathode without going through the circuit which includes the resistance R2. sired a choke 41 may also be provided which prevents the audio frequenc'y currents from passing to the cathode through the circuit including the resistance R2. Thus only the direct current component 1115 of the plate current is allowed to flow through R2.

Although I have provided the arrangement as recited above for keeping audio frequency currents out of the circuit involving R2, yet it is possible that small audio frequency currents may appear in this circuit. This is particularly true if the source of potential 33 is the output of a rectifier which is not perfectly filtered. The voltage of such currents will be taken up in a major degree by the drop in the choke 4'! and in the resistance 32. Thus a very small audio frequency voltage may appearacross the terminals of R2. Since it is clear that this audio frequency potential bears no relation to the strength of the signal to be compensated for, it is desirable to eliminate the effect of-this drop across the resistance R2. The arrangement of the'condenser 23 and the resistance 23 effectively eliminates even this slight effect. The condenser 23 is of fairly large value so that .it offers very little impedance to audio frequency currents. The resistance 23', however, is of a large value so that its impedance is great as compared to that of the condenser 23. The circuit comprising the condenser 23 and the resistance 23 and the circuit comprising the I resistanceRz afford two parallel paths for the passage of audio frequency currents in the circuit in which they are placed. The audio frequency drop acrossRz will of course be equal to the total audio frequency drop across 23 and the condenser 23. Since, however, the impedance of the resistance 23' is large with respect to that of the condenser 23, practically the entire drop across these two elements will appear across the resistance 23. Thus the magnitude of the audio frequency drop across the resistance 23 .is substantially equal to the audio frequency drop across the resistance R2. It will be seen that the resistance 23 is connected in such a manner in the circuit of the grid ID that its audio frequency drop is directly subtracted from the audio frequency drop appearing across the resistance R2. Thus any audio frequency voltage appearing across R2 is directly compensated for by a similar audio frequency voltage appearing across the resistance 23'. The condenser 4! is necessary to prevent the effect of the resistance 23 from being shortcircuited by the ground connection IS. The resistance 40 is provided in order that the total resistance of the path across the condenser 4| is If de- 1 high enoughto prevent the passage of an appreciable amount of radio frequency current. The radio frequency currents are provided with a return path from the plate to the cathode by means of'the condense-r i l. Of course the frequency of the variation in the direct current component of the plate'current, due to the variation in signal strength which we are attempting to compensate, is so slow that its effect on the condenser 23 is practically that of a direct current. Therefore the presence of the circuit comprising the resistance 255 and condenser 23 across the resistance R2 does not affect the regulating property of the resistance B2 on the bias of the grid 1 9.

,In addition to eliminating the effects of the audio frequency drop across the resistance R2, it is desirable that the length of time which it takes for th regulator to respond to changes in signal strength impressed on the grid 16 shall be made comparatively large For example,

when tuning the set to the desired signal, the

tuning elements should be adjusted so that the set will respond in maximum degree to the desired signal. Under these conditions the value of the signal will usually be well above the point at which an appreciable volume regulation is obtained. Thus a considerable variation in signal strength in either direction can occur without affecting the volume output of the set. If the volume control were to respond too quickly as the operator varied his condensers for purposes of tuning, the volume output of the set would remain constant over a fairly wide band. This would be due to the fact that as the tuning approached the resonance condition of the set, a certain predetermined Value of signal strength would be reached at which the volume control would start to operate. As the tuning more nearly approached the resonant point, the signal strength would increase, but the response of the volume control would be rapid enough to follow the rate at which said point was approached.

This action would continue as the tuning passed the resonant frequency point until the signal decreased on the opposite side of said point to the predetermined value at which; the volume control no longer exerted an appreciable effect. Thus it would be very difiicult for the operator to determine the resonant point for a particular frequency. However, if the rate at which the volume control responds were made slow enough, the operator could tune the set to its maximum volume output and the response of the volume control would not be rapid enough to follow the rate at which the adjustment of the tuning elements took place. I make the response of my volume control very slow by making the time constant of the circuit, comprising the resistances 22, 23' and the condenser 23, large. Since the time constant of such circuit depends upon the value of resistance therein, I make this constant large by making the value of the resistance in the circuit large. Since resistance 22 is placed directly in the plate circuit of the device l3, any drop across this resistance decreases the plate voltage of said device. Therefore, this resistance R2 is limited in actual practice to comparatively low values. However, the value of resistance 23 in no way affects the plate voltage of the device 13. Therefore, this resistance may be made as large as is desirable. I therefore select a value of R2 which will not aifect the plate voltage to an undesirable extent, and make the value of the resistance 23' large enough so that the time constant of its circuit is quite large. This time constant may be, for example, about. one-twentieth of a second.

Also as pointed out above, the larger the value of resistance 23, the smaller need be the value of the capacity of condenser 23. With the circuit arranged as above, any variations in output of detector 2 which occur at a rate greater than that determined by the time constant of the regulator circuit will not affect to an appreciable degree the value of the current 2'10 flowing through R2. Therefore, this device will respond only to Very slow changes in signal strength which is the result desired. Of course it is clear that the value of the time constant of the regulating circuit can be adjusted by adjusting the value of the resistance 23'.

The value of the signal received by a set is often of such a low value that it is desired to amplify this signal to the full extent of the set. Furthermore, an increase in magnitude of such a weak signal will not produce an excessive output in the set. Therefore it is desirable to arrange the device as described above so that it will not exert regulation upon signals below a certain value. This is exactly what I accomplish by another feature of my present invention which is incorporated in the arrangement as'shown in Fig. 1. In the analysis as given above, the entire variation in the voltage V10 0n the grid it was attributed to the change in 1'16.

This is substantially true if the signal received by the set is above a certain value. At signal values, however, below this value, the bias on grid it, due to the drop in R1 and R2, is comparatively small. As is evident from the analysis in the early part of the specification, the value of the direct current component is of the current of plate 9 flowing in the circuit of plate 8 under these conditions is comparatively large. Due to the fact that the radio frequency currents are bypassed by the condenser i l, practically the only portion of the current of plate 9 that will flow through the circuit, including the resistance R1, will be the direct current component 2'9. The current 2'10 through R1 consists of the sum of the currents 2'9 and 2'11. 1'11 is practically constant and depends primarily upon the resistance 32 and the value of the voltage source 33. I select the values of the constants of my device so that at signals below a certain intensity, current is forms a substantial part of the current 2'10. Upon the strength of the signal increasing, the grid ID becomes more negative, as shown above, and the current is decreases. At low values of signal strength, this decrease in the current is will produce an appreciable decrease in the value of 2'10. From Equation 3 it will be noted that decrease in 210 will tend to produce a decreasein the negative bias V1'0 as applied to the grid H3. At the same time, however, the increase in signal strength will cause 16 to decrease. The decrease in 16 will tend to cause an increase in V10. Thus it will be noted that the variations in 2'16 and 2'10 operate on the bias of grid iii in opposite directions. The constants of the circuits may be so chosen that at a predetermined value of signal strength, the effects of these two currents practically balance one another so that very little effect is had upon the signal received. This signal is therefore allowed to be amplified almost to the full capacity of the set. However, as the strength of the signal increases, the magnitude of V10 and consequently the magnitude of is becomes smaller. Since 2'11 remains practically constant, 2'9 at bermaintained by my device.

'certain higher values of signal strength no longer forms a substantial part of the current. 'iio.

Therefore, the variations in the value of is under "these conditions will not produce appreciable variations in the value of 2'10. At such higher values of signal strength, practicallyfth'e entire regulating effect is obtained through the variations of 2'15. Thus the regulation will not become effective to an appreciable degree until a predetermined value of signal received is reached.

The arrangement as described above will tend to produce a constant levelof output from the set. Since it is often desirable to adjust this level to suit varying conditions and varying tests, I have provided means whereby a particular level may be selected, which level will automatically plished by meansofthe tap 43 on the resistance 42 in the audio stage 3. By varying the position ofthe tap 43 on the resistance 42, any desired proportion of the total voltage variations appearing on the plate I5 and consequently the condenser 23 may be selected. If the tap 43 is placed at the upper end of the resistance 42, the total output of the detector stage 2 will be am plified and the maximum output will be obtained in the set. However, as the tap 43 is moved down along the resistance. 42, only' a portion of the output voltage of the detector control is not distributed by this latter arrangement. Any volume level which is selected by adjusting the tap 43 will automatically be maintained by my regulating arrangement.

, In Fig. 2 I have illustrated a slightly modified form' of my invention. In this figure I have applied the same reference numerals to those elements which are identically the same as those shown in Fig. 1. I have illustrated an additional 'radio frequency amplification stage. 50

which is exactly the same as'stage I. This is interposed between the antenna circuit 4 and the stage I, and is coupled to said circuits by means 'of coils 5| and 52, respectively. Stage 50 includes a tube 53 including a cathode, anode and control electrode. The cathodes of tubes 53 and! are connectedtogether by means of the conductor 54 while the ends of the inductance elements 5| and 5 opposite the grids of their respective tubes are connected tigether by means of a conductor 55. It willbe noted that in Fig. 2 the effect of the two radio frequency amplification stages actson the remainder of the circuit exactly the same as does the single radio frequency stage illustrated in Fig. 1. The current it is the sum of the direct current components of the plates of both tubes 53 and I while the varying bias of the regulating device is impressed upon the grids of both of said tubes.

The magnitude of signal voltage at which the regulating device becomes effective is fixed in the arrangement, as shown in Fig. l, by the values of the resistances as originally chosen. It is often desirable to regulate the value of signal voltage at which the regulation becomes appreciable. I accomplish this result by means of a simple expedient as illustrated in Fig. 2.

nected directly to the point 38 are connected by This is accom- The cathodes of tubes 53 and 1 instead of being cona 5 means of a variable tap 51 to any desired point on said resistance 56. In Fig. 2 I have shown the tap 57 as being set in a predetermined position. The amount of resistance 55 between the tap15l and the point 38 is indicated as R3, and the value of said resistance between the tap 51 and the point intermediate the resistances 22 and 23 is indicated as R4. It will be noted that current is divides at tap 51, a certain amount 2'12 flowing through resistance R13, and the remaining portion. of it which is indicated as 2'13 flowing through R4. Current in; which fiows through Ral'lOW consists of the sum of 215 and 2'13. Any decrease in the total current 2'16 increases the negative bias on the grids'of the radio frequency amplifiers as explained in relation to Fig. 1. Current 1'10 now consists of the sum of in and 2'12. Any decrease in 10 causes adecrease in the negative bias on the grids of the radio frequency amplifiers. By dividing the current is in the manner indicated, it is clear that variations in 2'12 and variations in'z'ia act on the grid bias of the radio frequency amplifiers in opposite directions. By a'djustingthe position of tap 51,,the relative proportion of it which flows through Rs and R4. may be regulated. Since-the ineffectiveness of the regulator. at low values;of*

signal voltage dependslargely on the proportion.

of the current is which fiows through the re sistance R1, it will be seen that by adjusting the proportion of is which flows through said'rec'esses, the value of signalvoltage at which the preciable is likewise controlled.

regulating effect of, the ,devicebecomes ap- The invention is not limited to the'particular' details of construction-as described above,- ,as many equivalentswill suggest themselves to those skilled in the art. For example,--any type of space discharge device, among whichmay be tubes of the screen-grid type,- may be used instead of the three-element tubes as shownin the drawing,

Also .various' different types of circuits may be used in the set instead of the particular one asv illustrated. It is accordingly desired that the ap-.

pended claims be given a broad interpretation commensuratewith the scope. of the invention withinthe art. a

a What is claimed is: l

F 1.- A carrier wave device, including a radio-frequency amplifier having a tube containing a cathode, an anode, and a control grid, a detector, means for causing the output of said detector to decrease with an increase in the modulation component and in the strength of the the carrier wave, a source of potential for said receiver, a voltagedividing impedance across said source, said amplifier and detector being connected to said voltage-dividing impedance so that the output of said amplifier and detector fiow through portions of said voltage-dividing impedance, an additional impedance inthe output of said detector, said additional impedance being connected to said voltage-dividing impedance to obtain a regulating voltage which is the difference between the drop across a portion of said voltage-dividing impedance and said additional impedance, and means associatedwith said last-named impedances and said control grid to impress on said control grid a negative bias which increases with an increase in the strength of the carrier wave only above a certain predetermined strength.

2. A carrier wave device, including a radio-frequency amplifier, a detector, means for causing the output of said detector to vary with variations in the modulation component and in the strength of the carrier wave, a source of potential for said receiver; a voltage-dividing impedance across said source, said amplifier being connected to said voltage-dividing impedance so that its output flows through a portion of said voltage-dividing impedance, an additional impedance in the output of said detector, said additional impedance being connected to said Voltage-dividing impedance to obtain a regulating voltage which is the di-fierence between the voltage drop across a portion of said voltage-dividing impedance and said additional impedance, and means associated with said last-named impedances'and said amplifier to impress 'upon said amplifier a volume control voltage which varies in accordance with the strength of the carrier wave only above a certain predetermined strength.

3. An automatic volume control device including a radio-frequency amplifier having a tube in which the discharge is controlled by a control grid, a detector, means for causing the output of said detector to vary with the strength of the carrier wave, an impedance, connections from the output circuits of said amplifier and detector to portions of said impedance for causing the'output of said amplifier and detector to iiow through said portionsof said impedance, an additional impedance 1 in' the output circuit of said detector, said additional impedance being connected to said firstnamed impedance to obtain a regulating voltage which'is' the difference between the drop across a ing a radio-frequency amplifier having a tube in which the discharge is controlled by a control grid,

a detector, means for causing the output of said detector to vary' with the strength of the carrier pedance to. obtain a regulating voltage which is the difference between the. drop across a portion oi said first-named impedance and said additional impedance, and means associated with said lastnamed impedances and said control grid to impress on said control grid a negative bias which increases with an increase'in the strength of the carrier wave only above a certain predetermined strength.

5. An automatic volume control device including a radio-frequency amplifier having a tube in which the discharge is controlled by a control grid, a detector, means for causing the output of said detector to vary with the strength of the carrier wave, a source of potential for said receiver, a voltage-dividing impedance across said source, said amplifier and detector being connected to said voltage-dividing impedance so that the output of said amplifier and detector flow through portions of said voltage-dividing impedance, an additional impedance in the output of said detector, said additional impedance being connected to said voltage-dividing impedance to obtain a regulating voltage which is the difierence between the drop across a portion of said voltage-dividing impedance and said additional impedance, and means associated with said last-named impedances and said control grid to impress on said control grid a negative bias which increases with an increase in the strength of the carrier wave only above a certain predetermined strength.

6. An automatic volume control'device including a radio-frequency amplifier, a detector, means for causing the output of said detector to vary with variations in the strength of the carrier wave, a source of potential, an impedance across said source, said amplifier being connected to said impedance so that its output flows through a portion of said impedance, an additional impedance in the output of said detector, said additional impedance being connected to said first-named impedance to obtain a regulating voltage which is the difference between the voltage drop across a po tion of said first-named impedance and said additional impedance, and means associated with said last-named impedances and said amplifier to impress upon said amplifier a volume control voltage which varies in accordance with the strength of the carrier wave only above a certain predetermined strength.

' JAMES R. NELSON. 

